1. Field of the Invention
This invention relates to a metal melting furnace equipped with a preheating tower which has shelf-plates and is arranged to have the material to be melted preheated with an exhaust gas within the preheating tower and more particularly to an arrangement for saving fork-shaped plates on which the melting material is to be placed within the preheating tower from quickly becoming unserviceable through wear and tear.
2. Description of the Prior Art
Aluminum metal melting furnaces are used for melting aluminum secondary alloys at aluminum die casting works, etc. Burners are used for melting. The melting process produces an exhaust gas at a very high temperature. It is known to utilize this high temperature exhaust gas for preheating the melting material. An example of this arrangement has been disclosed in Japanese Patent Publication No. SHO 58-30520 (Japanese Laid-Open Patent Application No. SHO 56-108092) entitled "Metal Melting Furnace with Preheating Tower". In the preheating tower of the metal melting furnace disclosed, there are provided a plurality of fork-shaped shelf-plates. The fork-shaped shelf-plates are inserted through two opposed side walls of the preheating tower into the inside thereof. The fore ends of the fork parts of the plates are thus arranged to abut on each other. The material which is placed on the fork parts under this condition is moved from one stage of plates to another by allowing it to drop off an upper stage of the plates down to a lower stage by moving the plates away from each other toward the opposed walls. Burners are ignited for the melting operation with a material such as ingots, scraps or the like placed on these shelf-plates.
After the aluminum material to be melted is heated within the main body of the metal melting furnace, a high temperature gas of the burners melts the material which has been preheated and placed on a dry hearth. An exhaust gas which has participated in melting on the dry hearth ascends from the shelf-plate disposed at the bottom of the preheating tower through the inside thereof before it is discharged from the upper part of the tower into the atmosphere. The preheating operation is carried out by heating the ingots or scraps on the shelf-plates with the exhaust gas during the process of its ascent.
During the melting operation, the molten aluminum must be kept at a high degree of temperature between 700.degree. and 750.degree. C. To meet this requirement, a high temperature combustion gas of 1200.degree. C. obtained by burning the burners are blown at the surface of the molten metal to raise the temperature thereof. An exhaust gas resulting from this process is applied to the melting material on the dry hearth to melt it at the bottom of the preheating tower. Following that, a preheating process is carried out by allowing the exhaust gas to acend the preheating tower. During the preheating process, the temperature of the exhaust gas gradually cools off until it becomes a low temperature gas. The low temperature gas is discharged into the atmosphere from the upper end opening of the preheating tower.
The combustion exhaust gas is thus used for melting and preheating the material. In melting, the gas must be at a sufficiently high temperature. The exhaust gas, the temperature of which is slightly lowered through the melting process, is required to remain at a sufficient degree of temperature for preheating without melting the material placed on the shelf-plates for preheating. For this purpose, temperature is measured at suitable parts of the preheating tower such as bottom and upper parts of the tower and the burner combustion is controlled according to the result of the measurement.
Normally, the exhaust gas after melting on the dry hearth is at temperature between 800.degree. and 900.degree. C. when it reaches the lowest shelf-plate of the preheating tower. After preheating, the exhaust gas is at temperature between 200.degree. and 250.degree. C. when it is discharged from the upper part of the preheating tower.
The shelf-plates made of carbon steel in general are burnt and damaged through exposure to the high temperature exhaust gas over a long period of time. In the event of any burnt damage occurring during the melting operation, the material such as ingots or scraps cannot be kept in place any longer. The material then falls on the dry hearth located at the bottom of the preheating tower without having been sufficiently preheated. The insufficient preheating causes an increase in the length of time required for melting. As a result, melting efficiency drops. The melting capacity lowers. The produced amount of molten metal decreases. An adequate casting operation then might eventually become impossible. To prevent such a burnt damage of the shelf-plates, it is conceivable to use heat resisting steel in place of the carbon steel. However, in cases where small pieces of scraps are mixed in the material, the small pieces tend to melt on the shelf-plates before larger pieces of the material are adequately preheated, because the exhaust gas flowing to the lowest shelf-plate is at a high degree of temperature between 800.degree. and 900.degree. C. The molten aluminum material then reacts with the heat resisting steel which is forming the shelf-plates. In that case, the plates are eroded by this reaction until they are damaged to about the same degree as in the case of carbon steel. The service life of the fork-shaped shelf-plates is not only affected by the temperature but also affected by the size and the kind of the material to be preheated. Therefore, for stable operation of the shelf-plate type preheating tower, it is necessary to make some arrangement to prevent the shelf-plate from being exposed to an excessively high temperature and to prevent the aluminum melting material from melting on the plates.
Let us first consider a temperature condition for preventing the aluminum material from melting on the lowest shelf-plates. Such temperature is about 500.degree. to 650.degree. C. although it depends on the kind and shape of the material and a period of time for which the material is to be kept on the shelf-plates. To arrange the lowest plate passing temperature of the exhaust gas to be within the above-stated range, it is conceivable to forcedly cool the exhaust gas by means of a heat exchanger or to have a low temperature external air mixed therewith as a cooling medium. However, the forced cooling necessitates additional facilities such as a heat exchanger, etc. and is not recommendable in terms of energy saving. As for mixing a low temperature external air as a cooling medium for reducing the temperature not only results in a loss of energy but also accelerates exidation of the aluminum material preheated by a mixed gas obtained after a large amount of oxygen of air is mixed in the exhaust gas. As a result, the aluminum would be oxidated and consumed thereby. The latter method is, therefore, not recommendable in terms of resource saving.